Method and apparatus for compensating for chrominance saturation

ABSTRACT

A method of color saturation compensation in a video signal is disclosed. The method includes the steps of: processing a luminance signal component of the video signal; determining whether the processing of the luminance signal results in a change in chrominance saturation; if the step of determining reveals that the chrominance saturation has changed, applying a chrominance compensation signal to the chrominance signals to counteract the effects of chrominance saturation, wherein the step of applying a compensation signal include the steps of: generating a compensation signal which is dependent on hue and luminance ratio (output luminance signal/input luminance signal). Apparatus for performing the method is also disclosed.

The present invention relates to a method and apparatus for use in videoprocessing systems to compensate for colour saturation, particularlyafter luminance processing has been performed.

BACKGROUND TO THE INVENTION

The human eye is able to interpret very many different colours. Colourmay be considered to be the combined effect of three distinct componentsor properties of light: hue, saturation and brightness. Hue is the nameof the colour, and places the colour in its correct position in theelectromagnetic spectrum. Saturation is the degree of intensity, orstrength, of a colour. Brightness describes differences in the intensityof light reflected from, or, transmitted by, a colour image.

In colour television systems, video signals are represented by threecomponents: Y (luminance), and two chrominance components (Cb, Cr). Thesignal is thus referred to as YCbCr. The luminance component (Y)contains the brightness information, and the chrominance components (Cb,Cr) contain the colour information.

The YCbCr system is defined in CCIR601, which relates to a worldwidedigital video standard. In an 8-bit digital system, Y has a normal rangeof 16 to 235, and Cb and Cr have a range of 16 to 240, with 128equalling 0.

The YCbCr colour space is represented graphically in FIG. 1. The valuesof Y, Cb and Cr are shown without the offsets described above i.e. therange of Y is 0 to 219, and the range of Cb and Cr is −112 to 112. Inthe colour space as illustrated, the hue is represented by the phaseangle with respect to the Cb axis. The magnitude of the chrominancecomponents vector, in combination with the luminance Y, provides ameasure for colour saturation

DESCRIPTION OF THE PRIOR ART

In prior art video processing systems, many quality enhancements areapplied to the luminance component. Linear and non-linear mappingfunctions are applied to the raw luminance signal to yield an improvedluminance value and so, an improved picture. However, this process leadsto a perceptual change in the viewed colour saturation. This can resultin images which are not optimal. Prior art compensation systems operateby adjusting the gain of the chrominance signals to offset the change incolour saturation. Specific solutions use the change in gain in theluminance signal to derive a corresponding change in the gain to thechrominance signals.

Image quality improvement by linear or non-linear processing of theluminance signal leads to a noticeable change in the output coloursaturation, which directly affects the quality of the viewed image.Increasing the luminance signal alone results in a decreased coloursaturation rendering viewed colours somewhat pastel in appearance.Decreasing the luminance signal alone results in increased coloursaturation rendering viewed colours more unnaturally vivid, which insome cases can appear over-saturated and uncomfortable to watch.

The problem with adjusting image quality by processing the luminancecomponent alone means that a degree of post processing of thechrominance signals is generally required in order to compensate forcolour saturation changes.

FIG. 2 shows a prior art solution to the problem of processing theluminance component. The input luminance signal Y_(in) is received by aluminance processor 101, which outputs a processed luminance signalY_(out). Y_(in) and Y_(out) are also fed into a gain generator 102 whichdetermines the amount of gain applied to Y_(in) and adjusts the inputchrominance signals Cb_(in) and Cr_(in) accordingly, in multipliers 103and 104, respectively, to yield Cb_(out) and Cr_(out).

A problem with such a compensation system is that it does not accountfor human perceptions of different hues, and treats all hues equally.

The present invention aims to provide a method and apparatus whichaddresses and ameliorates the problems described above.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method of coloursaturation compensation in a video signal, including the steps of:processing a luminance signal component of the video signal; determiningwhether the processing of the luminance signal results in a change inchrominance saturation; if the step of determining reveals that thechrominance saturation has changed, applying a chrominance compensationsignal to the chrominance signals to counteract the effects ofchrominance saturation, wherein said step of applying a compensationsignal include the steps of: generating a compensation signal which isdependent on hue and luminance ratio (output luminance signal/inputluminance signal).

Preferably, said hue is determined according to the relationship:Hue=tan⁻¹(Cr _(in) /Cb _(in))where Cr_(in) and Cb_(in) are the input chrominance signals.

Preferably, the hue may be approximated by the following steps: dividingthe absolute value of the smaller of Cr_(in) and Cb_(in) by the largerof Cr_(in) and Cb_(in); determining in which of eight 45° regions ofCbCr space a present sample resides by processing sign bits of Cb, Crand the difference between the absolute values of Cb and Cr; limitingthe resultant value such that it lies in the range 0 to 360 as a validhue.

Preferably, the smaller of the absolute value of Cb or Cr is divided bythe larger of the absolute value of Cb or Cr, wherein the result of thedivision is processed such that the result of the division process isaugmented such that it lies in the correct region according to the signof Cb, the sign of Cr and the sign of |Cb|-|Cr|.

Preferably, the compensation signal is derived from hue information,said hue information forming an input to a weighting factor calculationprocess which calculates a weighting factor for a particular hue, saidweighting factor being multiplied by the luminance ratio to yield thecompensation signal.

Preferably, said compensation signal is constrained so a not to exceed amaximum allowable gain.

Preferably, said weighting factor calculation process includescalculating a weighting factor described a linear approximation to anon-linear function.

Preferably, the linear approximation takes the form of a plurality ofline segments defining a relationship between hue and weighting factor.

Preferably, the plurality of line segments includes six discrete linesegments having relating hue to weight according to the table of FIG.14.

Preferably, the weighting factor may be modified by an amount determinedby a user-defined quantity.

According to a second aspect of the present invention, there is providedapparatus to perform the method and associated features of the firstaspect of the present invention.

Embodiments of the present invention apply a compensation gain to modifythe chrominance components, Cb and Cr, based on a particular hue.Embodiments are able to compensate colour saturation on bothover-saturated and de-saturated cases.

In a preferred embodiment, the hue-based compensation gain signal isobtained by multiplying the ratio of the output and input luminance witha hue-based weighting factor which varies depending on the hue. Thehue-based weighting factor is indicative of the relationship between theratio of changed saturation and the hue of the corresponding sample.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to understandhow the same may be brought into effect, the invention will now bedescribed by way of example only, with reference to the appendeddrawings in which:

FIG. 1 shows a representation of YCbCr colour space;

FIG. 2 shows a prior art approach to chrominance saturationcompensation;

FIG. 3 shows an overview of an embodiment of the invention;

FIG. 4 shows a graph plotting chrominance saturations at two luminancevalues;

FIG. 5 shows a graph plotting the relationship between ratios of changedsaturations and hue;

FIG. 6 shows a plot of a hue-based weighting function;

FIG. 7 shows a functional block diagram of the present invention;

FIG. 8 shows the relationship between chrominance components and hue;

FIG. 9 shows eight defined regions in CbCr space;

FIG. 10 shows a table detailing the eight regions of FIG. 9 togetherwith their sign values and hue calculations;

FIG. 11 shows a block diagram of the internal structure of a hueestimator;

FIG. 12 shows a block diagram of the internal structure of themultiplexer of FIG. 11;

FIG. 13 shows the linear approximation of the weighting function as sixindividual line segments; and

FIG. 14 shows a table listing the gradients and offsets for the sixlines of FIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 shows a general overview of an embodiment of the presentinvention. This embodiment utilises the chrominance and modifiedluminance signals to enhance the chrominance saturation. The inputluminance signal Y_(in) is received by the luminance contrastenhancement circuit or process 201 and modified according to knownluminance enhancement techniques. Such techniques may include using aluminance mapping function to enhance the luminance signal. The skilledman will be aware of such techniques. The modified luminance signal,Y_(out), is the output from circuit or process 201. FIG. 3 also shows acolour saturation compensation circuit or process 202, which receives asinputs Y_(in) and Y_(out) as well as the chrominance input signalsCr_(in) and Cb_(in).

The colour saturation compensation circuit or process may be arranged asa hardware circuit in custom or general circuitry, or may be arranged tooperate a software process on a suitably programmed or configuredprocessor, such as a DSP.

Circuit or process 202 operates on its 4 input signals to create twooutput signals Cb_(out) and Cr_(out) which are colour saturationcompensated chrominance signals. The output chrominance signals areessentially generated by multiplying the input chrominance signals witha hue-based compensation gain, the derivation of which is describedhere.

At an overview level, the present invention may be described by thefollowing equations:Cb _(out)=CGain*Cb _(in)   (1)Cr _(out)=CGain*Cr _(in)   (2)CGain is the compensation gain which may, in turn, be represented by theequation:CGain=Weight(hue)*Y _(out) /Y _(in)   (3)Weight(hue) is the hue-based weighting factor, the derivation of whichwill shortly be described.

As a result of the luminance enhancements performed on the luminancesignal, the resulting colour saturation can be acceptable,over-saturated or de-saturated. Embodiments of the present inventionseek to address the problems caused by over-saturation andde-saturation.

In the case of de-saturation, the compensation gain is limited in therange of:1.0<CGain<MaxGainwhere MaxGain is the maximum allowable compensation gain.

In the case of over-saturation, the compensation gain is limited in therange of:0<CGain<1.0

In order to take advantage of perceptional differences in coloursaturation as it applies to different hues, a hue-based weightingfactor, Weight(hue), is derived to improve compensation gain accordingto the hue of a given sample. Different colours have different ratios ofchanged saturation when their luminance changes. In this context, theratio of changed saturation is defined as the ratio of colour saturationafter luminance processing to colour saturation before luminanceprocessing.

FIGS. 4 and 5 are intended to illustrate the relationship between theratios of changed saturation and hue in a particular example. FIG. 4shows Hue as a value (0–360) on the x-axis, and saturation on they-axis. The graphs show two lines representing different saturationvalues for a given hue, for different luminance values. The dashed linerepresents a larger luminance value than the solid line. It can be seenfrom FIG. 4 that for a fixed luminance value, different hues havedifferent saturations. When the luminance value of a colour increases,its saturation decreases, as can be inferred by the dashed line sittingbelow the solid line in FIG. 4.

FIG. 5 shows a curve plotting ratio of saturations on the y-axis againstHue on the x-axis. This curve illustrates that when the luminance ischanged, its saturation will be changed, but that the ratio of changedsaturation is dependent on the particular hue. For instance, in thisexample, the ratio of changed saturation of ‘red’ (hue=100) is largerthan the ratio of changed saturation of ‘yellow’ (hue=177). For thisreason, different compensation gains are required for different hues.

Embodiments of the present invention provide a hue-based weightingfunction, fweight(hue), which relates a particular hue to acorresponding weighting factor. The function is derived from the curveof the relationship between the ratio of changed saturation and hue, asis shown in FIG. 5.

FIG. 6 shows an example of the function curve using certain experimentalvalues. The final weighting factor can be adjusted by a user-definedparameter WGain. The adjusted weighting factor is described by thefollowing equation:Weight(hue)=WGain*(fweight(hue)−1)+1   (4)

FIG. 7 shows a detailed view of an embodiment of the invention. It'smain feature is an enhanced detailed view of the colour saturationcompensation block 202 of FIG. 3, which is shown within the dotted box.

The embodiment of FIG. 7 receives three input signals: Y_(in), Cb_(in)and Cr_(in). It generates three output signals: Y_(out), Cb_(out) andCr_(out). The luminance processing is performed by circuit or process201.

A circuit entitled ‘Luma Ratio’ 301 is provided which receives as inputsY_(in) and Y_(out). It is operable to divide Y_(out) by Y_(in) to yieldLGain, which is the luminance ratio. This ratio, LGain is compared incomparator 302 with ‘1’. If LGain is larger than ‘1’ then the luminancehas increased, and colour saturation has consequentially decreased. IfLGain is less than ‘1’ then the luminance has decreased, and oversaturation detector 310 is used to determine whether the colour isover-saturated or not.

An OR gate 308 is provided which is operable to select an input ofmultiplexer 309. OR gate 308 is arranged to output a ‘1’ in the eventthat LGain is >1 and the colour is de-saturated or if LGain is <1 andover-saturation is detected. In either of, these events, the inputchrominance signals will require compensation.

The compensation signal CGain is applied to a first input of themultiplexer 309, and ‘1’ is applied to the other input. If the output ofOR gate 308 is 1, then the CGain input of the multiplexer 309 isselected. This has the effect of applying the CGain signal tomultipliers 311, 312 which each act on the input chrominance signals toproduce the output chrominance signals.

If the output of OR gate 308 is ‘0’, then the ‘1’ input to multiplexer309 is selected, applying ‘1’ to the multipliers 311, 312, with the neteffect that the output chrominance signals are unchanged from the inputsignals.

The over-saturation detector 310 is operable to detect whether aparticular colour sample is over-saturated or not, If the value of thesample's saturation is >1 then the colour is over-saturated. Otherwise,the colour is not over-saturated. The degree of colour saturation can becalculated in RGB space according to the following equation:Saturation=(Max(R, G, B)−Min(R, G, B))/Max(R, G, B)   (5)where R, G and B are red green and blue signals respectively. To convertYCbCr signals to RGB signals, the following equations may be used:

$\begin{matrix}\left. \begin{matrix}{R = {Y + {1.366*{Cr}}}} \\{G = {Y - {0.700*{Cr}} - {0.334*{Cb}}}} \\{B = {Y + {1.732*{Cb}}}}\end{matrix} \right\} & (6)\end{matrix}$

Since the value of Max(R, G, B) in (5) is always positive, the detectionof over saturation can be carried out by detecting the sign bit ofMin(R, G,. B). If it is negative, then the saturation value is >1 andthe colour is over-saturated. If the sign bit is positive, then thecolour is not over saturated.

In order to calculate CGain for a particular hue, it is necessary tocalculate a hue value from the input chrominance components. Huecalculator 303 receives as inputs Cb_(in) and Cr_(in). Using Cartesiancomponents, the hue is related to the chrominance components as shown inFIG. 8. Cb and Cr are shown as mutually perpendicular axes. The hue isgiven by the phase angle of the chrominance vector, with respect to theCb axis, as shown. The magnitude of the vector in combination with theluminance is a measure of the colour saturation. The hue can becalculated using the following equation:Hue=tan⁻¹(Cr/Cb)   (7)where tan⁻¹ is the arctan function. Equation (7) returns the realinverse tangent of Cr/Cb in the range 0 to 360° using the signs of botharguments to determine the quadrant of the returned value. Normally, foran inverse tangent function, tan⁻¹(x), when its independent variable, x,is ranged from 0 to 1, the function can be approximated by a linearfunction: tan⁻¹(x)≈x*180/π and is ranged from 0 to 45°.Thus, therequired angular information can be extracted from the quotient definedby |Cr|/|Cb| (when |Cr|<|Cb|), or |Cb|/|Cr| (when |Cr|>|Cb|) combinedwith the sign information from Cr and Cb.

According to this, the the four quadrants of CbCr colour space can bedivided into 8 regions of 45°. Each region can be identified by the signbits of Cb, Cr and (|Cb|−|Cr|). The hue angle in each region can bedefined by the quotient of |Cr|/|Cb| (when |Cr|<|Cb|), or |Cb|/|Cr|(when |Cr|>|Cb|). For example, in FIG. 9, region (1) is defined by Cband Cr both being positive with |Cb|>|Cr|. The quotients of |Cr|/|Cb|correspond to the tangents of angles from 0 to 45°. As the chrominancesignals move into region (2), |Cb|<|Cr| and Cb and Cr remain positive.The quotients of |Cb|/|Cr| correspond to the angles from 0 to 45° withrespect to the +Cr axis.

Similarly, as the chrominance signals traverse each quadrant, the valuesrepresented by the quotients correspond to angles in the range 0 to 45°to 0, because only the magnitudes of Cb and Cr are applied to thedivider and the smaller value is always divided by the larger. The tableof FIG. 10 indicates the regions, the range of hue angles, the quotientvalues, the sign bits of Cb and Cr and |Cb|−Cr| samples in therespective regions. In the Sign column, ‘0’ represents positive and ‘1’represents negative. The final column, ‘Hue’, illustrates a simplemethod of determining hue from the sign bits and the quotient values.

FIG. 11 shows a schematic of the internal structure of the huecalculator 303 shown in FIG. 7. The chrominance input signals Cb_(in)and Cr_(in) are respectively applied to absolute value circuits 801, 802which output only the magnitude of the input signals. |Cb|, the outputof circuit 801 is passed to subtraction circuit 803 as subtrahend. |Cr|,the output of circuit 802 is passed to subtraction circuit 803 asminuend. The output of circuit 803 equals the difference between the twoinput signals, i.e. |Cb|=|Cr|. The sign bit of the calculation indicateswhich input signal to the subtractor 803 has the largest magnitude.

Switching circuit 804 receives |Cb| and |Cr| as major inputs and asignal indicative of the sign of |Cb|-Cr| as a control input. Thecontrol input ensures that the switch is operative such that the largerinput signal is always directed to the following divider circuit 805 asdivisor, and the smaller input signal is always directed to the dividercircuit 805 as dividend.

Divider circuit 805 acts on its input signals to generate a quotientvalue which is input to a multiplexer 807. The internal construction ofmultiplexer 807 is shown in FIG. 12. The multiplexer 807 receives as aninput the output of the divider 805. This input signal is augmented inmultiple adder units to generate a series of possible angle values whichare all fed into translator 901, which is in effect a furthermultiplexer. The control inputs to translator 901 are derived from threedigital signals representing the signs of Cb, Cr and |Cb|−|Cr|. Thesesign bits are decoded in logic decoder 806 to create the three controlbits required to select one of the eight inputs to translator 901. Ineffect, the circuit of FIG. 12 puts the results of the table of FIG. 10into effect.

The output of multiplexer 807 is fed into a limiter 808 to ensure thatthe eventual Hue output is limited in the range 0 to 360.

Referring to FIG. 7, the output of the Hue calculator 303 is next passedto a Weight Calculator 304 which has as a further input WGain. Weightcalculator 304 implements weighting function, fweight(hue). As can beseen from FIG. 6, fweight(hue) is a non-linear function which has 360independent variables. To avoid the hardware complexity required byusing a look up table to store all results for all 360 variables, thenon-linear function is approximated using six line segments. Theapproximation is shown in FIG. 13. Each line in the approximation isdefined by the equation:fweight(hue)=K*hue+B   (8)where K is the gradient of the line and B is the offset.

The parameters of the six lines (L1–L6) are shown in the table in FIG.14. Using the function's value fweight(hue) and user-defined WGain, thehue based weight(hue) can be calculated using equation (4).

The ideal compensation gain CGain is obtained by multiplying theadjusted weight, weight(hue), by the luminance gain, LGain, atmultiplier 306. The maximum gain estimator 305 is arranged to output amaximum allowable gain, MaxGain, to limit the range of the compensationgain which may be applied. This avoids any danger of colour change. Inan 8-bit digital system, chrominance signals are defined to have a rangeof 16–240, with 128 being equal to 0. The maximum value of thechrominance components is 112. Thus, MaxGain can be defined by theequation:MaxGain=112/max(abs(Cb _(in)), abs(Cr _(in)))   (9)where max(abs(Cb_(in)), abs(Cr_(in))) is the maximum value of(abs(Cb_(in)) and abs(Cr_(in)), and (abs(Cb_(in)) and abs(Cr_(in)) arethe absolute values of Cb_(in) and Cr_(in) respectively.

Embodiments of the present invention may be realised in software using asuitable programmed processor, especially a DSP. Alternatively,embodiments may be realised in hardware using either discrete componentsor, preferably, a custom integrated circuit, such as an ASIC.

The present invention includes and novel feature or combination offeatures disclosed herein either explicitly or any generalisationthereof irrespective of whether or not it relates to the claimedinvention or mitigates any or all of the problems addressed.

1. A method of colour saturation compensation in a video signal,including the steps of: processing an input luminance signal componentof the video signal to generate an output luminance signal component;determining whether the processing of the input luminance signalcomponent results in a change in chrominance saturation; generating acompensation signal based upon a hue and a luminance ratio equal to theoutput luminance signal component divided by the input luminance signalcomponent; and if the step of determining reveals that the chrominancesaturation has changed, applying the compensation signal to chrominancesignals components of the video signal to counteract the effects of thechrominance saturation change.
 2. The method of claim 1, wherein saidhue is determined according to the relationship:hue=tan⁻¹(Cr _(in) /Cb _(in)) where Cr_(in) and Cb_(in) are thechrominance signal components of the video signals.
 3. The method ofclaim 2, wherein the hue may be approximated by the following steps:dividing the smaller of the absolute value of the Cr_(in) and Cb_(in)components by the larger of the absolute value of the Cr_(in) andCb_(in) components to generate a quotient input signal; determining inwhich of eight 45° regions of CbCr space the Cr_(in) and Cb_(in)components reside by processing a sign bit of the Cb_(in) component, asign bit of the Cr_(in) component, and a sign bit of the differencebetween the absolute values of the Cb_(in) component and the absolutevalue of the Cr_(in) component; and limiting the approximated hue to liein a range of 0 to 360 as a valid hue.
 4. The method of claim 3, whereinthe quotient input signal is augmented to generate a plurality ofpossible hues, the approximated hue selected from the plurality ofpossible hues based upon the sign bit of the Cb_(in) component, the signbit of the Cr_(in) component, and the sign bit of (|Cb_(in)|−|Cr_(in)|).5. The method of claim 1, wherein generating the compensation signalfurther includes calculating a weighting factor for the hue, saidweighting factor being multiplied by the luminance ratio to yield thecompensation signal.
 6. The method of in claim 5, wherein saidcompensation signal is constrained so as not to exceed a maximumallowable gain.
 7. The method of claim 5, wherein calculating theweighting factor includes calculating the weighting factor based upon alinear approximation to a non-linear weighting function.
 8. The methodof claim 7, wherein the linear approximation takes the form of aplurality of line segments defining a relationship between a variablehue and a value of the weighting function.
 9. The method of claim 8,wherein the plurality of line segments includes six discrete linesegments relating the variable hue to the value of the weightingfunction according to the following table: Line Segment Hue rangeGradient Offset L1  0–52 −0.0014 1.2314 L2  53–100 +0.0002 1.1525 L3101–177 −0.0028 1.4797 L4 178–235 +0.0015 0.7513 L5 236–280 −0.00031.1637 L6 281–360 +0.0023 0.4163


10. The method of claim 5, wherein the weighting factor may be modifiedby an amount determined by a user-defined quantity.
 11. An apparatusconfigured to compensate colour saturation in a video signal,comprising: means for processing an input luminance signal component ofthe video signal to generate an output luminance signal component; meansfor determining whether the processing of the input luminance signalcomponent results in a change in chrominance saturation; means forgenerating a compensation signal based upon a hue and a luminance ratio,the luminance ratio equal to the output luminance signal componentdivided by the input luminance signal component; and if the means fordetermining reveals that the chrominance saturation has changed, meansfor applying the compensation signal to chrominance signal components ofthe video signal to counteract the effects of the chrominance saturationchange.
 12. The apparatus of claim 11, including a processor.
 13. Theapparatus of claim 12, wherein the processor is a Digital SignalProcessor (DSP).
 14. The apparatus of claim 11, including a customintegrated circuit such as an ASIC.
 15. A colour saturation compensationcircuit for compensating chrominance signals based upon a hue-basedweighting factor, comprising: a hue calculator configured to determine ahue, based upon a sign bit of a first chrominance signal, a sign bit ofa second chrominance signal, a sign bit of an absolute value of thesecond chrominance signal subtracted from an absolute value of the firstchrominance signal, and a chrominance quotient; and a weight calculatorconfigured to determine the hue-based weighting factor based upon thehue and a hue-based weighting function.
 16. The colour saturationcompensation circuit of claim 15, wherein the chrominance quotient isthe absolute value of the second chrominance signal divided by theabsolute value of the first chrominance signal if the absolute value ofthe second chrominance signal is less than the absolute value of thefirst chrominance signal, or the absolute value of the first chrominancesignal divided by the absolute value of the second chrominance signal ifthe absolute value of the first chrominance signal is less than theabsolute value of the second chrominance signal.
 17. The coloursaturation compensation circuit of claim 15, further comprising: a lumaratio module configured to determine an luminance gain based upon aninput luminance signal and an output luminance signal; an oversaturationdetector configured to determine if colour saturation of a video signalis over-saturated, the video signal comprised of the output luminancesignal, the first chrominance signal, and the second chrominance signal;and a first multiplier configured to multiplying the luminance gain andthe hue-based weighting factor to determine a compensation gain.
 18. Thecolour saturation compensation circuit of claim 17, further comprising:a second multiplier configured to multiply, when the colour saturationof the video signal is over-saturated, the first chrominance signal andthe compensation gain to generate a compensated first chrominancesignal; and a third multiplier configured to multiply, when the coloursaturation of the video signal is over-saturated, the second chrominancesignal and the compensation gain to generate a compensated secondchrominance signal.
 19. A method for compensating chrominance signalsbased upon a hue-based weighting factor, comprising: determining a hue,based upon a sign bit of a first chrominance signal, a sign bit of asecond chrominance signal, a sign bit of an absolute value of the secondchrominance signal subtracted from an absolute value of the firstchrominance signal, and a chrominance quotient; and determining thehue-based weighting factor based upon the hue and a hue-based weightingfunction.
 20. The method of claim 19, wherein the chrominance quotientis the absolute value of the second chrominance signal divided by theabsolute value of the first chrominance signal if the absolute value ofthe second chrominance signal is less than the absolute value of thefirst chrominance signal, or the absolute value of the first chrominancesignal divided by the absolute value of the second chrominance signal ifthe absolute value of the first chrominance signal is less than theabsolute value of the second chrominance signal.
 21. The method of claim19, further comprising: determining an luminance gain based upon aninput luminance signal and an output luminance signal; determining ifcolour saturation of a video signal is over-saturated, the video signalcomprised of the output luminance signal, the first chrominance signal,and the second chrominance signal; and determining a compensation gainby multiplying the luminance gain and the hue-based weighting factor.22. The method of claim 21, further comprising: if the colour saturationof the video signal is over-saturated, generating a compensated firstchrominance signal by multiplying the first chrominance signal and thecompensation gain; and if the colour saturation of the video signal isover-saturated, generating a compensated second chrominance signal bymultiplying the second chrominance signal and the compensation gain. 23.A colour saturation compensation circuit for compensating chrominancesignals based upon a hue-based weighting factor, comprising: a means fordetermining a hue, based upon a sign bit of a first chrominance signal,a sign bit of a second chrominance signal, a sign bit of an absolutevalue of the second chrominance signal subtracted from an absolute valueof the first chrominance signal, and a chrominance quotient; and a meansfor determining the hue-based weighting factor based upon the hue and ahue-based weighting function.
 24. A colour saturation compensationsystem configured to receive two input chrominance components of aninput video signal, an input luminance component of the input videosignal, and a processed luminance component of an output video signal,and to compensate the two input chrominance components to generate twooutput chrominance components of the output video signal, comprising: aluma ratio module configured to process the input luminance componentand the processed luminance component to determine a luminance ratio; anover-saturation detector configured to process the input chrominancecomponents and the processed luminance component to determine if coloursaturation of a colour sample is over-saturated, the colour samplecomprised of the input chrominance components and the processedluminance component; a hue calculator configured to process the inputchrominance components of the input video signal to generate a hue; aweight calculator configured to process the hue to generate a hue-basedweight; a first multiplier configured to multiply the hue-based weightand the luminance ratio to generate a compensation signal; and a secondmultiplier to multiply the input chrominance components of the inputvideo signal and the compensation signal to generate the two outputchrominance components of the output video signal, if the coloursaturation of the colour sample is over-saturated.